The anatomy of sloths

Everybody loves sloths, and whenever we talk about sloths we have to remember that the two living kinds (Bradypus – the four species of three-toed sloth – and Choloepus – the two species of two-toed sloth) are but the tip of the iceberg when it comes to sloth diversity. This article – an excerpt from Naish (2005) (though with citations added that were absent in the published article) – briefly reviews the anatomy of fossil sloths, though there are references to the living forms where appropriate.

A typical fossil sloth can be imagined as a rather bear-shaped, shaggy-furred mammal with particularly powerful forelimbs, a barrel-shaped ribcage, a stout tail, prominent curved hand and foot claws and a markedly broad, robust pelvis.

Skulls from various sloths. a. Eremotherium, a Pleistocene megatheriid known from both South and North America. b. Nematherium, a mylodontid from Early Miocene South America. c. Scelidotherium, a scelidotheriine mylodontid from the Pliocene and Pleistocene of South America. d. Thalassocnus, a nothrotheriid from the Miocene and Pliocene of the Pacific coast of South America. e. Acratocnus odontrigonus, a choloepodine megalonychid from the Pleistocene of Puerto Rico. f. Acratocnus simorhynchus, an even shorter-snout choloepodine, from the Pleistocene of Hispaniola. Not to scale. Images by Darren Naish, redrawn from various sources. From Naish (2005).

Sloth skulls are diverse in form and range from the deep and broad, snub-faced morphology seen in Bradypus and some megalonychids to the elongate almost horse-like skulls of megatheriids and others (Gaudin 2004). Some megalonychids had a domed cranium resulting from marked enlargement of the sinuses within the frontal bones. The sloth palate is rugose and covered in pits and grooves and there are distinctive deep laminae that descend ventrally from the pterygoid bones (Gaudin 2004). The tip of the sloth mandible is usually spout-shaped and there is a foramen, representing an external opening of the mandibular canal, on the side of the lower jaw. In sloths with particularly long-rooted teeth there is a distinct bulge on the ventral margin of the lower jaw.

Sloths have peculiar teeth. They do not possess deciduous teeth but have a single set of high-crowned, open-rooted teeth (Bargo et al. 2006) that grow continuously throughout life, and the lack of a replacement dentition has made it difficult to homologise sloth teeth with those of other mammals. Incisors are absent, and it is not really possible to distinguish between the similar premolars and molars. The living tree sloth Choloepus, as well as some mylodontids, megalonychids and nothrotheriids, possess caniniform teeth separated from the other teeth by a diastema. The upper caniniforms of these sloths are ahead of the lower caniniforms and, while some evidence suggests that the upper caniniform in Choloepus is a true canine, this probably isn’t the case for the lower caniniform. In the Pleistocene megalonychid Megalocnus from Cuba, and in certain other genera, the two most anterior upper jaw teeth have been described as ‘pseudorodentiform’ and are more incisiform than caniniform.

Sloth teeth lack enamel and are composed instead of two different kinds of dentine plus an outer layer of cementum, the softer dentine forming the innermost region of the tooth. When sloth teeth erupt they are devoid of the cusps and basins seen normally in mammalian teeth and are simple and cylindrical in form. As the teeth occlude against those in the opposite jaw, valleys and cusp-like structures are formed as the two kinds of dentine erode differentially (Naples 1989, 1995). Some fossil sloths had squarish or subrectangular teeth and, in these forms, transverse ridges between the valleys are particularly prominent.

Arms, hands and hips

Arm of the mylodontid Glossotherium from Pleistocene South America. Image Darren Naish, from Naish (2005).

The forelimbs of most sloths are about subequal in length to the hindlimbs, the most prominent exceptions being the long-armed tree sloths of the genus Bradypus. Mylodontids had a particularly prominent olecranon process on the ulna. Recent studies have shown that the length of the olecranon process relative to the rest of the ulna is a good indicator of digging ability in mammals as the olecranon provides the attachment area for the triceps, the main muscle used in digging. Forelimb bone strength in mylodontids was also high and shows that the forelimbs were resistant to impact with the ground (Bargo et al. 2000). Furthermore, the wide, straight and relatively flat claws of these sloths resemble those of living mammals that dig. Accordingly, mylodontids seem to have been proficient diggers that unearthed roots and tubers and they may even have constructed burrows.

Sloths are amazingly diverse and unusual in hand morphology. Among megatheriids, primitive species of Eremotherium were pentadactyl (albeit it with a short thumb and a fifth digit with only one phalanx) while the advanced species E. laurillardi was tridactyl, possessing only digits III-V, and of these only digits III and IV had unguals (Cartelle & De Iuliis 1995).

Some of the diversity present in sloth hands. L to r: Pleistocene mylodontid Glossotherium robustum, in which the hand is pentadactyl; Pleistocene megatheriid Eremotherium laurillardi, in which digits I and II have been lost; Pleistocene megatheriid Megatherium americanum, in which the thumb is absent. Not to scale. Drawings by Darren Naish, redrawn from various sources.

Bradypus, a taxon that’s notable for being outside the clade that includes the majority of other sloth lineages (Gaudin 2004, Pujos et al. 2007), possesses only digits II-IV on the hand, and the megalonychid Choloepus only has II and III. Several sloth groups exhibit fusion of various manual phalanges, including of both phalanges in the thumb (in Eremotherium) and of the two phalanges at the base of the third digit (in Thalassocnus), as well as fusion of metacarpals to carpals.

The sloth pelvis is massive and broad and unusual in that the ischia are connected to the vertebral column (in most tetrapods only the ilia are connected), a feature that sloths share with all other xenarthrans with the sole exception of Cyclopes, the Pygmy anteater. The femur in fossil sloths varies from robust to very robust with the femora of giant megatheriids being shaped like a wide rectangle. The tibia in most fossil sloths is proportionally short and is also massively constructed. As is true of the hand, some sloth groups reduced the number of toes with only three present in some megatheriids.

Mummified sloth skin preserved in the arid caves of Chile, Argentina, Arizona and Nevada provides excellent information on ground sloth skin and fur. Small bony ossicles were embedded in the skin of the mylodontids Mylodon, Glossotherium and Paramylodon, and probably also in Eremotherium, but are definitely not present in the mummified skin of Nothrotheriops. The fur itself was either yellowish or reddish brown.

Locomotion and posture

The configuration of the ground sloth foot and ankle indicates that most of these animals were plantigrade (that is, they placed the entire surface of the foot on the ground). However, it was argued as early as the 1840s that at least some ground sloths walked with a pedolateral foot posture: that is, with most of the weight supported by the outer margins of the feet. This bizarre configuration meant that the dorsal surface of the foot faced laterally.

Right foot of the giant megatheriid Eremotherium. Though this is how the foot looks in lateral view: the lateral surface of the foot is the anatomical dorsal surface. Only digits III-V were present in this sloth, and of these only digit III possessed a claw (this was curved toward the midline, and its tip is thus not visible in lateral view). Image by Darren Naish, from Naish (2005).

The centre of gravity in the ground sloth body and the strength of their hindlimb bones, pelvis and vertebrae indicate that at least some forms could walk bipedally. Fossil trackways confirm this. Most sloths have hands and hand claws that appear well suited for the manipulation of foliage and the robust tail seen in most fossil sloths suggests that they may have sat in a tripodal posture when foraging and eating. The tripodal abilities of ground sloths have proved inspirational to palaeontologists working on other fossil tetrapod groups.

Living tree sloths are good swimmers so it seems reasonable to assume that ground sloths were too. However, a few fossil sloths reveal morphological features which indicate that they were habitual, rather than occasional, swimmers and amphibious habits have been suggested for both scelidotheriine mylodontids and nothrotheriids. One group of nothrotheriid seems to have been truly semi-aquatic (Muizon & McDonald 1995, Muizon et al. 2003, 2004).

And - - seeing as this is another article on Cenozoic South American megafauna, I’m sure you’re wondering how it’s going with that montage I featured here back in July’s toxodont article. Here’s the answer... (still working on it: a larger version will be uploaded to my deviantART gallery later today)...

- ., McDonald, H. G., Salas, R. & Urbina, M. 2003. A new early species of the aquatic sloth Thalassocnus (Mammalia, Xenarthra) from the Late Miocene of Peru. Journal of Vertebrate Paleontology 23, 886-894.

Pujos, F., de Iuliis, G., Argot, C. & Lars, W. 2007. A peculiar climbing Megalonychidae from the Pleistocene of Peru and its implication for sloth history. Zoological Journal of the Linnean Society 149, 179-235.

POSTSCRIPT: how could I write about sloths and not include this? ...

The views expressed are those of the author(s) and are not necessarily those of Scientific American.

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ABOUT THE AUTHOR(S)

Darren Naish

Darren Naish is a science writer, technical editor and palaeozoologist (affiliated with the University of Southampton, UK). He mostly works on Cretaceous dinosaurs and pterosaurs but has an avid interest in all things tetrapod. His publications can be downloaded at darrennaish.wordpress.com. He has been blogging at Tetrapod Zoology since 2006. Check out the Tet Zoo podcast at tetzoo.com!

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